1. Field of the Invention
The present invention relates to a differential oscillator circuit incorporated in a semiconductor device. More particularly, it relates to a differential oscillator circuit wherein the operating current of an oscillator and the operating current of a buffer amplifier are made common.
2. Background Art
In recent years, semiconductor devices used for portable gear have been required to have lower current consumption. Because of this, a method has been used frequently which reduces current consumption as a whole by adopting a stack structure, in which plural circuits having different functions are vertically stacked, to make operating currents for the circuits common. In oscillator circuits as well, a configuration shown in FIG. 4 has been well known as a circuit in which an oscillator proper and a buffer amplifier are driven by a common current.
FIG. 4 is a circuit diagram for showing a configuration of a related art oscillator circuit having a buffer amplifier. In FIG. 4, reference numeral 45 denotes a common-collector Colpitts-type oscillator. Reference numeral 44 denotes a commom-emitter buffer amplifier. Reference numeral 43 denotes an output terminal. Reference numeral 46 denotes a coupling capacitor. Reference numeral 47 denotes a decoupling capacitor. Reference numerals 55, 56, 57, and 58 denote a resistor respectively.
The oscillator 45 is the Colpitts oscillator circuit wherein a capacitor 49 is connected between the base and emitter of a bipolar transistor 48, a capacitor 50 is connected between the emitter and collector of the bipolar transistor 48, and a capacitor 51 and an inductor 52 are connected in series between the base and the collector. The oscillator 45 takes the form of collector common, and the collector of the bipolar transistor 48 is grounded via the decoupling capacitor 47.
The buffer amplifier 44 is a common-emitter amplifier, and the emitter of a bipolar transistor 53 is grounded via the decoupling capacitor 47 and have an inductor 54 as a load.
An Oscillation signal output from the oscillator 45 is input via the coupling capacitor 46 into the buffer amplifier 44 and is output from the output terminal 43.
In the circuit shown in FIG. 4, a common operating current flows into the bipolar transistor 48 included in the oscillator 45 and the bipolar transistor 53 included in the buffer amplifier 44.
However, since this configuration effects single-ended operation, this oscillator circuit is sensitive to noise conveyed via its power source and substrate. In consequence, the oscillator circuit has the drawback of a tendency to cause unstable oscillation operation.
For this reason, a technique using a differential circuit which effects balanced operation has been proposed as an oscillator incorporated in semiconductor devices (see, for example, non-patent literature 1).
FIG. 5 is a circuit diagram showing the configuration of a differential oscillator disclosed in the non-patent literature 1. This differential oscillator comprises a pair of field-effect transistors. In FIG. 5, reference numeral 12 denotes a differential oscillator. The differential oscillator 12 takes the form of a differential oscillator circuit as follows: the drain of the field-effect transistor 18 is connected to the gate of the field-effect transistor 19, and the drain of the field-effect transistor 19 is connected to the gate of the field-effect transistor 18; both ends of a resonator 14 comprised of an inductor 16 and a capacitor 17 in parallel are connected to the respective drains of the field-effect transistors 18 and 19; the sources of the field-effect transistors 18 and 19 are connected to the drain of a field-effect transistor 20; the gate of the field-effect transistor 20 is biased via a bias terminal 15 to govern an operating current which flows into the differential oscillator 12; and the midpoint of the inductor 16 is provided with a power supply voltage Vcc.
The non-patent literature 1 is as follows: Aparicio, R. and Hajimili, A., “A Noise-Shifting Differential Colpitts VCO”, IEEE J. Solid-State Circuits, December 2002, Vol. 37, pp. 1728 to 1736 (FIG. 1).
However, since oscillators using the differential circuit shown in FIG. 5 do not include a buffer amplifier, the problem of increased current consumption is caused, so that such oscillators are required to include the buffer amplifier.